A two-stage nucleation model with incubation effect for hydride precipitation in zirconium alloys

Hydrogen accumulates in zirconium alloy cladding tubes during reactor operation as a by-product of waterside corrosion. When its concentration exceeds the solid solubility (TSS), plate-like hydrides precipitate, which may lead to embrittlement. The severity of embrittlement depends on hydrogen concentration and hydride morphology, including orientation, characteristic length, and connectivity of the hydride network. While hydrogen uptake is mainly governed by alloy composition and operating conditions, hydride morphology is strongly influenced by the post-operation thermal and mechanical history. Thermomechanical cycles during handling and storage can cause repeated dissolution and reprecipitation, promoting long, radially oriented hydrides that facilitate brittle crack propagation. Consequently, hydride-induced embrittlement becomes critical during handling and dry storage of spent nuclear fuel. Accurate prediction of hydride morphology requires a proper description of nucleation kinetics. In our previous approach, nucleation was modeled using classical theory; however, validation against recent experimental data showed excessive sensitivity to uncertainties in hydrogen concentration and solubility under certain thermal conditions. To overcome this limitation, a two-stage kinetic model with a finite incubation period has been developed. This concept is based on the formation of metastable precursor phases preceding the stable δ-hydride. The incubation period represents the time needed for hydrogen accumulation and local lattice rearrangement prior to stable nucleus formation. The present work introduces this model and demonstrates its effect on predicted hydride precipitation behavior.